The number of young people who spend their lives locked in their bedrooms has tripled in the past two years to as many as 18,500, a welfare group's estimate shows.

Hong Kong Christian Service's previous estimate compiled from a survey of 36 secondary schools in March 2005 put the number at 6,000.

This extreme form of social withdrawal is more common in Japan, where it is called hikikomori. Japan is believed to have a million sufferers.

In Hong Kong, the welfare group warned the latest figure was only the tip of the iceberg because most cases did not come to light until their families sought help.

But the group says it may have to axe its programme for young people - the city's only scheme - to help these urban hermits because of a lack of government funding.

The latest estimate was derived from calculations based on the 192 cases the group handled between November 2004 and last month, and a survey of secondary schools conducted from July to September last year.

Of these young hermits, some had isolated themselves for as long as six years, with the youngest being just 12.

An analysis of the 192 cases showed about 80 per cent would kill time surfing the internet or playing computer games. Fifty-two per cent would watch television and 39 per cent would idle away the hours in a dark corner at home.

Characterised by their lack of self-esteem and self-confidence, these young people usually did not perform well at school, and almost 60 per cent only had Form Three qualifications at most.

Christian Service principal co-ordinator Winnie Ying Fung-sau yesterday said these cases could lead to domestic violence, in the worst scenario, and early detection of the problem was important.

Ms Ying attributed the phenomenon partly to the increasing competition at school and pressure to succeed within the community.

'Many of them become evasive and eventually choose to lock themselves up in their bedrooms during the day and only came out to have food or take a shower,' she said.